In the manufacturing of paper, papermaker's machines are employed which typically comprise a forming section, a press section and a dryer section. The three sections of a typical papermaker's machine sequentially remove water from a paper furnish to form a paper sheet. In the forming section, the furnish, comprising about 75 to 80 percent by weight water, is deposited on a moving forming wire, and water is drained through the wire to form a still wet paper web or sheet. In the press section, the wet web, carried by a wet press felt, passes through one or more roller nips to further remove water. In the final dryer section, the web, oriented on a dryer felt, is pressed against steam heated cylinders to form a final sheet.
The felts or belts employed to transport the furnish and web through the papermaking machine must be particularly adapted to accommodate the unique conditions encountered in the various sections of the papermaking machine. Due to the difference in operating conditions between the sections, the felts for each section are generally manufactured to specific design parameters.
In the wet press section, where a large percentage of the water is removed, the properties of the felt are critical to the efficient operation of the papermaking machine. The materials and construction of a typical wet press felt have evolved as papermaking machines have been improved. Currently, papermaking machines may operate at speeds of up to 4,000 to 5,000 feet per minute. A wet press felt must be capable of operating continuously at such speeds while providing for maximum drying in order to minimize energy requirements in the dryer section and to minimize breakage of the sheet (sheet strength increases as the water content of the sheet decreases). Additionally, the felt must have a sufficient life to minimize costly down time of the machine for felt replacement. Currently, a standard wet press felt comprises a woven fabric base, either single or multi-ply, to which is affixed at least one layer of batt material comprising non-woven fibers. Typically, the batt layer is affixed to the fabric base by needling, an operation well known to a person of ordinary skill in the art. A variety of other constructions are sometimes employed to form wet press felts such as non-woven felts or felts having laminated layers.
The base fabric of a wet press felt must provide sufficient strength to allow high speed, extended operation while being resistant to compaction and provide adequate dewatering and air permeability properties. Also, the base fabric is preferably formed so as to have minimal effects on the smooth surface of the batt layers supported thereon.
Typically, wet press felts are woven as endless loops so that changes in overall felt length during the life of the felt are minimized. By weaving the felts as endless loops, the crimped warp yarns are oriented in the cross-machine direction when the felt is actually placed on the machine. Thus, possible stretching of the felts by "flattening" of the fabric crimp is minimized. Also, such endless loops avoid marking problems associated with seamed felts. However, such endless felts are difficult to weave and provide with batt layers and are difficult to install on papermaking machines.
Generally, the industry has not employed a wet press felt which is woven flat and joined by a seam. In a typical flat woven fabric joined by a seam into an endless felt, the machine-direction warp yarns are crimped during weaving. The elastic effect in such a felt, coupled with the possibility of product marking and weakness of the seam, are believed to have resulted in a general lack of use of seamed felts as wet press felts.
The wet press felt of the present invention is an improved wet press felt which is easier and more economical to form. Installation in a typical papermaking machine requires less down time due to the use of a seam, and the present invention provides a wet press felt which exhibits good operating life and physical properties.